Hello Deb - and everyone.
I haven't had the opportunity to read most of these yet, but there are quite a few other books on solar returns out there. There is an archive of the extensive former stock list of the (long defunct) Astrology Bookshop in London as it used to be before a change of management several years ago, which makes for very useful investigation when one is looking to learn up on a particular area of astrology by buying the classic texts on it!
http://www.astrology.co.uk/Books/b_SubjIndex.html - from there, click "Progressions and Returns", and you will see titles by Marc Penfield and Cynthia Bohannon in addition to the two already mentioned in this strand. However, whether or not either is an improvement on or adds anything worth paying the price of the book to Mary Fortier Shea's work I would rather doubt.
But there are two other modern in-print ones that are widely recommended: "Solar Returns Formulas and Analyses" by Nance McCullough, and 'Interpreting Solar and Lunar Returns" by Janey Stubbs and Babs Kirby
Additionally, there some classic texts on solar returns that are available second-hand or, in the first case, new, which I personally intend to investigate in the future, when I have the requisite time and money, including:
"Astrologia Gallica Book 23: Revolutions" by Jean-Baptiste Morin
"Interpreting Solar Returns" by James Eshelman (rare and expensive second-hand)
"Solar and Lunar Returns" by John Filbey (rare and expensive second-hand)
"The Technique of Solar Returns" by Alexandre Volguine (expensive second-hand, but still listed for sale new in the original French at a more affordable price from amazon.fr and fnac.com)
Then there is also the Project Hindsight volume "On Solar Revolutions, Part II" by Abu Ma'Shar. I'm not sure if any copies of this are left, and it's a pity that only Part II ever appeared. I was much too late in learning astrology at a more advanced level to sign up for the original Project Hindsight subscriptions, so I missed out on this - and have not seen any copies second-hand. Nonetheless, I'm much looking forward to Professor Charles Burnett's eventual publication of his work-in-progress translation of Abu Ma'Shar's "Great Introduction to the Science of Astrology", which Professor Burnett expects to be published in three or four years from now. But I honestly don't know whether Abu Ma'Shar's treatise on solar revolutions formed a part of the Great Introduction or was an entirely separate work!
Finally, siderealists champion a work by Donald Bradley on solar and lunar returns. This is likely to be the last on my list of priorties, even though it is supposed to be very good of its kind!
Well, I hope if nothing else that these additional titles lead anyone reading this who wants to know all there is to learn about solar returns to some further investigations!
Philip